# 9.2\. 限制 對“每個具體類映射一張表”（table per concrete-class）的映射策略而言，隱式多態的 方式有一定的限制。而`<union-subclass>`映射的限制則沒有那 么嚴格。 下面表格中列出了在Hibernte中“每個具體類一張表”的策略和隱式多態的限制。 **表 9.1\. 繼承映射特性(Features of inheritance mappings)** | 繼承策略(Inheritance strategy) | 多態多對一 | 多態一對一 | 多態一對多 | 多態多對多 | 多態 `load()/get()` | 多態查詢 | 多態連接(join) | 外連接(Outer join)讀取 | | --- | --- | --- | --- | --- | --- | --- | --- | --- | | 每個類分層結構一張表 | `<many-to-one>` | `<one-to-one>` | `<one-to-many>` | `<many-to-many>` | `s.get(Payment.class, id)` | `from Payment p` | `from Order o join o.payment p` | _支持_ | | 每個子類一張表 | `<many-to-one>` | `<one-to-one>` | `<one-to-many>` | `<many-to-many>` | `s.get(Payment.class, id)` | `from Payment p` | `from Order o join o.payment p` | _支持_ | | 每個具體類一張表(union-subclass) | `<many-to-one>` | `<one-to-one>` | `<one-to-many>` (僅對于`inverse="true"`的情況) | `<many-to-many>` | `s.get(Payment.class, id)` | `from Payment p` | `from Order o join o.payment p` | _支持_ | | 每個具體類一張表(隱式多態) | `<any>` | _不支持_ | _不支持_ | `<many-to-any>` | `s.createCriteria(Payment.class).add( Restrictions.idEq(id) ).uniqueResult()` | `from Payment p` | _不支持_ | _不支持_ |